This
page presents an interface to a Bluetooth
headset for
Amateur radio application. The key advantage in my solution is that the
Bluetooth headset's answer/hang up push-button can be used to control
the
radio PTT. This is very convenient inside and around the shack.

BACKGROUND

The enforcement of city and
provincial/state by-laws force the hams to use a hands-free microphone
as replacement to the classic hand mike.

The Jabra A210 interface is the best approach for add-on Bluetooth
capability to amateur radio transceivers. It is sold as an interface
for
non-Bluetooth capable cell phones, but can be quite easily adapted to
interface radio transceivers. The following hyperlinks will give you
the background behind interfacing to ham radio. I am very thankful to
those who did the early work:

There is one feature
in the Jabra A210 device that is not documented anywhere and that has
not been proposed by anyone so far for PTT support. It is the ability
for the A210 to pick up a cell phone call or hang up using the
Bluetooth handset push-button. I have discovered this after analyzing
the literature on wired headsets and on the A210 itself. With proper
interfacing, this feature can be used for wireless PTT support. In a
nutshell, the A210 sends a 400-millisecond short to ground on the phone
plug tip every time the headset push-button is depressed. This cannot
be sensed or perceived unless a weak pull-up resistor is populated on
that signal.

In order for this feature to be used for PTT support, a D-Type
Flip-Flop function must be added. Pressing once engages the PTT,
pressing again dis-engages the PTT, and so on. I have decided to
implement this function in a PIC micro-controller. The PIC12F683 is a
low cost 8-pin device; this is smaller than the 14-pin MC4013 dual
D-Flip-Flop chip. I also use an analog function of the PIC, its
comparator input, for better noise immunity and less loading on the
audio line. I have also added a timeout timer that will release the PTT
if it is engaged for more than 2.5 minutes. This could prove useful if
the bluetooth link is lost.

Since a 12V DC source is provided in a mobile application, I have added
a +3.7 V DC voltage regulation that will work with anything higher than
+7 V DC.

RELEASE
CONTROL

02/02/2010 - Added notes
specific to West Carleton Amateur Radio Club group project. Others may
disregard.

10/01/2010 - R3 and R4 had their
value swapped on the schematic and on the parts table. The PCB view and
PDF files have been updated.
05/01/2010 - Initial Release

SCHEMATIC
DIAGRAM

My Jabra A210 - Radio interface
schematic is shown on the right hand side. The circuit is quite
straightforward. The headset microphone audio
signal flows from left to right, from the Jabra A210 to the radio
microphone input. A coupling capacitor C1 is added to eliminate
possible DC contention on that line. The
headset earphone audio flows from right to left, so from the radio
speaker to the A210 earphone input. I have populated an optional
attenuator made of R3 and R4 to reduce the audio level going into the
A210.

Click
on
the
figure
to
enlarge
it.

Note that the speaker ground
and the headset microphone ground
are kept separate. Inside the Jabra A210, there are separate pads on
the
PCB. This is to keep any ground loop (hum) to minimum.
The pull up resistor that allows to produce the pulse to ground is R5.
The pulse is detected by the PIC micro-controller through R2. A wired
PTT push-button provides a local PTT function identical to the headset
push-button. The PTT output sent to the radio is provided by Q1, an NPN
open collector transistor. When the PTT is active, the transistor
shorts the radio PTT line to ground. There is also an optional PTT LED
line provided for visual acknowledgment of the PTT state.

PCB
LAYOUT

I have
created a PCB layout that custom-fits a Hammond 1551RBK enclosure. This
is a
neat and compact solution. The PCB layout is shown on the picture to
the
right.

The PCB is made of
single-sided copper clad glass-epoxy material of 0.062" thickness.

The copper
pattern resides on the PCB bottom side. The components are located on
the top side.

When hand-making the
PCB, drilling of
holes is required to solder the components to the copper pattern.

Click
on
the
figure
to
enlarge
it.

If
you wish to make this PCB yourself, here is the PCB copper layout
file. It prints on letter-size paper. When printing in full size (no
scaling) the dimensions and proportions should be accurate.

I wrote the PIC12F683 firmware in ANSI C. I used BoostC and the
BoostIDE environment. These are free compiler and integrated design
environment available from SourceBoost
Technologies. The code is commented so that you can understand what
is happening. Have a look at the .c file. Here is the whole design
directory.

Some
preparation is required in order to use the Jabra A210 Bluetooth
interface in my application.

Open up the unit. This is a delicate
task if you want to preserve the looks of your A210. Using a sharp
knife, carefully crack open the casing. It is easier to start on the
side where the casing has a long recessed slot. Work your way to the
corners and the ends. Go gentle. The glue should give pretty
easily. Watch out for the flying white button! Work over a table and
make sure to recover the white button that falls off when you spread
the two covers.

Remove the old battery. If you
intend to connect
the unit to the voltage regulator for external battery operation or
12V DC, you
must disconnect and remove the old battery. Simply cut off or desolder
the battery wires. There is hot-melt glue on the contacts so take that off
first. The
battery is also glued to the casing; use a small flat screwdriver
and gently
pull it off.

Remove the coiled signal cable.
Simply desolder or cut off the four wires attached to the PCB. There is
hot-melt glue on the contacts and wires so take that off first.
Preserve the rubber grommet to plug the cable hole after completion.
Simply cut off the
cable from the grommet with a sharp knife.

Implement
VE3LC's
modification.
The
A210 normally transitions to a power-saver mode after 60 seconds
without any audio fed into it. For an amateur radio application, we do
not want this. To keep it alive, install a 47K resistor as shown on
either of the pictures to the right. I recommend using a
surface-mounted resistor. I used an 0805-size resistor and some AWG-30
"wire-wrap" wire. Implement the solution you
are more comfortable with. In either case, the result is a 47K resistor
connected between pin one of that chip and ground. If you are still
unwilling to deal with surface-mounted components, implement VE3LC's
mod using a regular leaded resistor as shown in his
PowerPoint
presentation.

Click
on
the
figure
to
enlarge
it.

Click
on
the
figure
to
enlarge
it.

Attach new wiring.
Desolder any old pieces of wire from the PCB and connect new wires to
the six PCB pads shown on the picture to the
right. Use long enough wires to allow interconnecting with the
interface PCB inside the Hammond box. Using colored wires is a good
idea.

Drill a hole in the A210 to feed the wires
to the
Hammond enclosure. You want to send all the wiring to the
Hammond enclosure located underneath. Drill a hole through the bottom
cover that is big
enough to accommodate all the new wires. The best location is somewhere in the center of
the area where
the battery used to be located.

I
highly
recommend
using an i.c. socket in the PIC micro-controller location.

The
PCB is quite small, so I had to keep the pad size to minimum. This
makes re-work (replacing components) difficult without damaging the
pads and traces. Make sure that the components are in the right
locations before soldering!

Depending
on
the
electrolytic capacitor package used in C2, it may or may not fit
in the proposed enclosure when mounted vertically. Install C2 last and
bend its leads 90 degrees to mount it horizontally (parallel to the PCB
surface), and above the other components.

Install
the
PIC
micro into its socket only after the entire soldering process
is completed.

Install the momentary PTT push-button and
the PTT LED (optional) on the top face at one extremity of the
Hammond enclosure. Make sure that the PCB will still fit inside the
enclosure with the proposed push-button and LED locations. See the
picture to
the right for the proposed location. Note that I used a push-button
with built-in LED.

Drill a hole to feed the wires from the
A210 inside the enclosure. You want this hole to be located on
the top face of the enclosure, underneath the interface PCB. Position
that hole to
align with the hole on the bottom cover of the A210 so that the A210 is
neatly positioned on top of the enclosure. Use the
A210 bottom cover to mark the location on the enclosure for an exact
hole line up.
Once again see the picture on the right
hand side for a suggested A210 location.

Click
on
the
figure
to
enlarge
it.

Drill a hole on one side of the Hammond
enclosure to feed the DC power cable and the radio speaker cable.

Feed the A210 wires inside the enclosure.

Attach the A210 to the top of the Hammond
enclosure. I used Coax-Seal putty. Use something that can be easily removed. Velcro is also fine.
Remember that the Bluetooth pairing pin-hole is located on the bottom
cover of the A210!

Install the interface PCB. Bring
all the wiring underneath the PCB and above it by routing it to one of
the open corners in the Hammond enclosure.

Solder the wires and cables to the proper
locations.
Follow the wiring diagram shown to the right. If you do it right, you
should be able to take the PCB out of the enclosure when all
the wiring is still attached.The result should look similar to my
assembly shown below.

Install the proper connectors for the radio
interface. Follow your radio manufacturer's wiring instructions
or the radio schematic diagrams for the microphone connector pinout.

Click
on
the
figure
to
enlarge
it.

VERIFICATION AND
ADJUSTMENT

Make sure that everything visually checks OK. Do not connect the cables to the radio yet. Apply power to the unit. Repeatedly press the
PTT push-button. The PTT LED should alternate between on and off
states.
Press on the A210 power button until its blue LED flashes a few time.
From that point on the A210 LED should flash once every few seconds.

Now is the time to pair up the A210 and your headset. Using a
needle-sized object, press the pairing button in the hole under the
A210 casing. The blue LED should become solid lit. Now put your headset
in pairing mode. This is usually done by pressing the headset
push-button for about 10 seconds, but follow the headset instruction
manual.
The pairing should be completed in about 20 seconds. This will be the
case when the A210 LED returns to a single flash every few seconds. Now
turn off both the headset and the A210.

At this point, connect the radio speaker and the microphone connector.
Go to an unused frequency on your transceiver. If the SSB mode is
available, select it. This will reduce the RF interference you produce
when the PTT is engaged.

Now turn the A210 back on. Then turn on the headset. The radio should
transition to Transmit mode and the PTT LED will turn on. This is
normal and inherent to the Jabra A210 push-button operation. Release
the
PTT by either pressing the headset push-button once or pressing the
wired PTT
push-button on the Hammond enclosure.

As far as audio adjustments go, listen to an occupied frequency and
increase the speaker volume on the radio. You should hear the radio
speaker audio into the headset earpiece. For transmit audio, you need
to monitor yourself into another receiver or ask someone to help you.
There is a 3-position microphone slide switch on the A210. Select the
switch position for best transmitted audio.

OPERATION AND
PERFORMANCE

You are now operating your Bluetooth headset on
Amateur Radio! There are a few things you need to know when using your
headset on the air. Here they are:

There
is
a
one-second
delay
between
depressing the headset push-button and
the radio PTT action occurring. This delay is generated by the A210 and
there is nothing that can be done to reduce it. In a typical simplex or
repeater
conversation, this is not much of a problem, but forget about
contesting using the headset push-button! If you need fast PTT action, just use the wired PTT
push-button instead. Note that the headset
microphone audio is always present at the radio connector.

Powering
up
the
Bluetooth
headset
will
put the interface in transmit mode, the
LED will turn on
and so will the transmitter. Just remember to quickly revert to receive
by pressing on the PTT push-button once. Again, the A210 causes
this and nothing can be done to remove it...

I have verified that the
range is around 10m (30 feet) in line of
sight. Walls and floors will reduce the range though. So I would not
claim
that you can walk around everywhere inside the house and expect it to
work flawlessly. But "inside and around the shack" is probably a better
way to
describe the range.

I
have
noticed
that
there
may
be a low-level constant-pitch whining noise
present on the transmit audio. The intensity of that noise seems to be
dependent on the headset type used. It is quite low, certainly not
enough to annoy the people listening to you, but it may show up.

Once
you know these things, you can enjoy using your bluetooth headset on
any amateur radio transceiver like I do in the car. I actually like it
even better inside and around the shack. I can walk around and still
provide
input on our local VHF SSB weekly net without having to run to the
radio every time they turn it over to me. Very convenient!